There are a number of alternators and generators (hereinafter power producing devices) known by those skilled in the art for generating either AC or DC electrical power from a source of mechanical energy. One group of power producing devices employs electromagnetic field windings to establish the excitation magnetic field. The excitation magnetic field is the magnetic field that is established to generate electric power by means of rotating or passing conductive material across the magnetic field lines as is known to those skilled in the art. While these power producing devices generally are easy and inexpensive to manufacture, they generally are characterized by being less efficient and bulkier as compared to power producing devices that employ permanent magnets to establish the excitation magnetic field, a second group of power producing devices hereinafter permanent magnet (PM) devices.
The permanent magnet devices also are advantageous because power is not provided to the rotor and there are no ohmic losses on the rotor. As a result, there is no need for rotating windings and an exciter or brushes to communicate the electric power to the rotating windings to establish a magnetic field. This advantage is important for reliability and size of power producing devices in general. For example, in an environment where moisture, mud or dirt can be present, the elimination of exciters and brushes reduces the chance of electrical shorting and failure of the power-producing device. The lack of ohmic losses improves the overall efficiency and simplifies cooling of the power-producing device.
Although the permanent magnet devices have desirous characteristics, they do have shortcomings or design characteristics unique to this group of power producing devices. Typically there is little or no control of the field strength for the excitation magnetic field, and hence the terminal voltage, because the field is being produced by permanent magnets. As a result, the terminal voltage also will vary as a function of the rotational speed of the rotor, stator or load current and/or the operating temperature of the magnets.
Also, because of the low synchronous reactance and the fact that a permanent magnet generated field cannot be turned off, currents are typically very high if there is a short circuit fault. Further, permanent magnets are susceptible to demagnetization, if they operate against a strong opposing armature reaction or at too high a temperature. The magnetic characteristics of permanent magnets typically vary as a function of temperature and age, as such; power output and the terminal voltage will vary as a function of the temperature of the magnet as well as its age. This temperature dependency also imposes limits on the magnetic materials selected for use as a mechanism for minimizing the temperature effect.
There is shown in FIG. 1, an elementary schematic cross-sectional elevation view of a permanent magnet biased, brushless synchronous motor/generator 1 developed for a flywheel energy system. This motor/generator 1 includes a stator 2 and a rotor 3. The stator 2 includes a single stator winding 4 that is wound so it lies within the rotor 3 and between the two sections or ends 5a,b of the rotor.
At each end 5a,b of the rotor 3 is an alternation of magnetic steel poles 7 and permanent magnetic poles 6 where one end 5b of the machine is configured with all "north" permanent magnets and the other end 5a with all "south" permanent magnets. Also, the permanent magnet poles 6 at one end 5a are aligned with the magnetic steel poles 7 at the other end 5b. Further, at the end 5b with the "north" permanent magnetic poles 6, the magnetic steel poles 7 at that end become south poles and vice-versa for the other end 5a.
An electromagnetic field winding 8 is disposed between the magnetic steel poles 7 and the permanent magnet poles 6 and is configured to push flux axially. When the field winding 8 is not energized, each end 5a,b is independent. The flux being pushed axially also is pushed only through the salient magnetic steel poles 7 at either end 5a,b so as to either enhance (i.e., boost) or reduce (i.e., buck) the flux being produced by the permanent magnet poles 6.
The field windings 8 for the motor/generator 1 provide a means for adjusting or modulating the excitation flux so the magnitude of the terminal voltage remains essentially constant during variable speed conditions. However, this motor/generator 1 because of its configuration and design yields a rather complex flux path and a device that imposes limitations on axial length. Its complex design also makes its manufacture costly and time consuming.
It thus is desirable to have a permanent magnet power producing device in which the excitation field is adjusted or modulated to compensate for load current changes and/or variable speed inputs that drive the device as well as for temperature, wear and variations in the magnetic characteristics of the permanent magnets. It would be particularly desirable for such a permanent magnet device to be capable of effectively nullifying the magnetic field being produced by the permanent magnets so as to shut off the power-producing device. It also would be desirable to have a permanent magnet motor that is configured with permanent magnets and a means for adjusting or modulating the magnetic field being produced thereby to yield a variable speed motor. Such power producing devices and motors preferably would be simple in construction and assembly as compared to other such power producing devices and permanent magnet motors as are known in the art.